Dipole antenna arrangement for radio with separate speaker-microphone assembly

ABSTRACT

A speaker microphone assembly, used with a portable radio unit containing receiving and transmitting equipment for receiving and transmitting radio signals, includes a housing with a helical antenna element mounted to the top wall to form one element of a dipole antenna. A coaxial cable having inner and outer conductors is connected from the portable unit equipment to the speaker microphone housing. The center conductor is connected to the helical antenna and the outer conductor is terminated in the housing at a first point which is ground potential. A second outer conductor surrounds and is insulated from the first outer conductor, and is connected to the first outer conductor at the first point. The second outer conductor extends for a predetermined length along the first outer conductor to form the other element of the dipole antenna.

TJite States atet 1191 Gorcik et a1.

1 1March 13, 1973 1 DIPOLE ANTENNA ARRANGEMENT FOR RADIO WlTH SEPARATESPEAKER-MICROPHONE ASSEMBLY [73] Assignee: Motorola, lnc., FranklinPark, 111.

[22] Filed: Nov. 8, 1971 [21] App1.No.: 196,433

[52 11.5. C1. ..32S/16, 325/111, 325/118, 325/178, 343/841 51 1111.01...11104b1/04 [58] Field of Search ..325/l6,1l1,112,118,119, 325/178,361, 365', 343/841 [56] References Cited UNITED STATES PATENTS Knott etal ..325/178 X Litke ..325/118 Primary ExaminerBenedict V. SafourekAttorney-Vincent J. Rauner et a1.

[57] ABSTRACT A speaker microphone assembly, used with a portable radiounit containing receiving and transmitting equip ment for receiving andtransmitting radio signals, includes a housing with a helical antennaelement mounted to the top wall to form one element of a dipole antenna.A coaxial cable having inner and outer conductors is connected from theportable unit equipment to the speaker microphone housing. The centerconductor is connected to the helical antenna and the outer conductor isterminated in the housing at a first point which is ground potential. Asecond outer conductor surrounds and is insulated from the first outerconductor, and is connected to the first outer conductor at the firstpoint. The second outer conductor extends for a predetermined lengthalong the first outer conductor to form the other element of the dipoleantenna.

16 Claims, 4 Drawing Figures PATENTED MR 1 3 I975 SHEET 10F 2 2 5 2 a iI INVENTOR SI JOHN W. FALTZ STANLEY W. GORCIK CHARLES T. HILTON JR.

RICHARD A. KAMYSZ BY: W ATTY.

PATENTEUMAR 13 1975 3, 720.874 SHEET 2 OF 2 INVENTORSI JOHN w. FALTZSTANLEY w. GORCIK CHARLES T."HILTON JR. 7 RICHARD A. KAMYSZ BY: wax/x44ATTY.

DIPOLE ANTENNA ARRANGEMENT FOR RADIo WITH SEPARATE SPEAKER-MICROPHONEASSEMBLY BACKGROUND Speaker microphone assemblies currently employed,such as that described in U.S. Pat. No. 3,370,236, issued to R. H.Walker, and assigned to the same assignee, include an antenna elementmounted thereon for radiating the transmitted radio signals andreceiving radiated signals. Such assemblies have been used to advantagefor a number of years. However, the antenna configuration does notresult in a fully satisfactory radiation pattern because the assemblyitself does not provide a substantial ground plane for the antennaelement. Furthermore, the assembly does not isolate the antenna elementfrom the portable radio unit thus allowing the radiated signals toaffect the operation of the radio unit.

Although a speaker microphone assembly as described in'the above patentis designed to be held in the hand, doing so can cause a degradation inantenna efficiency due to the presence of the body capacitance of theuser.

Antenna elements used with assemblies such as that described above havea lower input impedance than the equipment with which they areassociated. In order to match the equipment to the antenna element,carefully selected discrete matching components must-be added to theassembly. The component selection and placement is critical, and theadded component and labor costs will increase the unit cost.

SUMMARY It is an object of this invention to provide a speakermicrophone assembly for a radio unit including an antenna mountedthereon for radiating and receiving signals, wherein the antennaconfiguration provides an improved radiation pattern.

Another object of this invention is to provide a speaker microphoneassembly having an antenna element thereon, which has structureproviding a substantial effective ground plane for the antenna element.

Yet another object of this invention is to provide a speaker microphoneassembly for a radio unit which has structure providing substantialisolation between the antenna element and the radio unit.

Still another object of this invention is to provide a speakermicrophone assembly having an antenna mounted thereon wherein theantenna performance and efficiency is not substantially degraded due tothe presence of the body capacitance of the user.

Another object of this invention is to provide a speaker microphoneassembly with an antenna mounted thereon which does not require discretecomponents for matching the antenna element to the radio equipment.

In practicing this invention, a speaker microphone assembly is providedfor use with a portable unit containing receiving and transmittingequipment for receiving and transmitting signals, wherein the assemblyis adapted to be held in the user's hand. The assembly includes ahousing having top, bottom and side walls, with transducer elements andequipment actuating switches mounted therein. Conductors couple thetransducer elements and switches to the radio equipment. A helicalantenna element is detachably mounted to the top wall of the housing andacts as one portion of a dipole antenna for radiating and receivingsignals. A coaxial transmission line having outer and inner conductorscouples the equipment to the speaker microphone assembly. The outerconductor is terminated at a first point in the assembly housing whichis the antenna feed point. The inner conductor is connected to theconnector which detachably mounts the antenna element. A second outerconductor surrounds and is insulated from the outer conductor of thecoaxial transmission line and is connected to the transmission lineouter conductor at the first point in the housing. This outer conductorextends for a predetermined length along the coaxial transmission line.The coaxial transmission line outer conductor and the second outerconductor act to form the second portion of the dipole antenna. As thehousing is located at the low impedance point or feed point of thedipole, it has a minimal effect upon the dipole antenna performance.

In an alternate embodiment, the housing walls are coated with aconductive material which is connected to the first point and acts toform the second portion of the dipole antenna.

THE DRAWINGS DETAILED DESCRIPTION Referring to FIG. 1, there is shown aportable radio unit 10 which contains an FM radio transmitter andreceiver, and a battery for operating the transmitter and receiver. Oneterminal of the battery (not shown) provides a reference or groundpotential. A speaker microphone assembly 11 is connected to the portableradio unit via a connector 12. Radiated radio frequency signals arereceived by an antenna including antenna element 22 on assembly 1 l andcoupled to the receiver in portable unit 10 where'they are demodulatedto produce audio signals. The audio signals are coupled from thereceiver in unit 10 through connector 12 to assembly 11 where they arereproduced by speaker 46, shown in FIG. 3, enabling the user to hear themessage. Audio messages to be transmitted are received at assembly 11where they are converted to electrical signals by microphone 45, shownin FIG. 3. The electrical signals are coupled through connector 12 tothe transmitter in radio unit 10 for modulating a radio frequencycarrier signal. The modulated radio frequency carrier signal is coupledfrom the transmitter in unit 10 through connector 12 to speakermicrophone assembly 11 where it is radiated by the antenna includingelement 22.

Speaker microphone assembly 11 includes a housing 15 which has a topwall 16, a bottom wall 17 and side walls 18. One of the side walls 18has a grillework l9 thereon for allowing audio to be conductedtherethrough from speaker 46 to microphone 45. The side wall oppositegrillework 19 (not shown) may be a removable cover which is secured tothe assembly with screws at 20. A helical antenna element 22 isdetachably secured to housing 15. Antenna element 22 includesa threadedrod 23 which mates with a connector 25, mounted to top wall 16 ofhousing 15. A cable 27 extends through bottom wall 17 of housing 15 andis connected to connector 12 for providing an electrical connectionbetween speaker microphone assembly 11 and portable radio unit 10.

Portable radio unit is adapted to be carried on the person in a locationsuch as on the users belt. Housing is adapted to be held in the usershand while he is using portable unit 10 for communicating with a remotepoint. It may also be supported on the users belt or shirt by anattachment to the removable cover such as is shown in patent No.3,370,236.

Referring to FIG. 2, there is shown a portion of cable 27 speakermicrophone assembly 11. Cable 27 includes a coaxial transmission lineconsisting of a center conductor 29, a dielectric insulating material30, a braided shield or outer conductor 31, and an outer insulation 32.Surrounding the coaxial transmission line and extending for apredetermined length therealong, is a second braided shield or secondouter conductor 34. Second outer conductor 34 will be explained ingreater detail in a subsequent portion of this application. Conductors36, 37 and 38 are also'provided in cable 27 for interconnecting thecomponents located in housing 15 with the equipment in portable radiounit 10. An outer insulating cover 39, formed of a plastic material, encases conductors 36, 37 and 38, second outer shield 34, and the coaxialtransmission line.

Referring to FIG. 3, antenna element 22 includes an antenna radiatingelement 41, shown in the cutaway section of antenna element 22.Radiating element 41 consists of a heavy gauge wire conductor and isformed into a helix to provide a helical antenna. The heavy gaugeconductor provides effective signal reception and transmission andallows antenna element 22 to be maintained in a semi-rigid condition. Anouter plastic jacket 42 surrounds radiating element 41, and acts toinsulate the element and assist in maintaining antenna 22 in asemi-rigid condition. Threaded rod 23 is attached to one end ofradiating element 41 and is shown inserted into connector in FIG. 3.

Helical antenna element 22 forms one portion of a dipole antenna whichis resonant at a particular frequency. A dipole antenna provides animproved radiation pattern over monopole antennas which have beenemployed. In the embodiment shown, the dipole antenna is resonant atapproximately 460 MHz. The physical length L of helical antenna element22 is selected such that its electrical length is approximately aquarter wavelength at the resonant frequency of the dipole antenna.

Housing 15 of speaker microphone assembly 11 has a microphone 45, aspeaker 46 and a switch 47 mounted therein. Speaker 46 includes a frame48 for supporting the speaker cone and mounting the loudspeaker in thehousing. Conductor 36 in cable 27 provides an electrical connectionbetween microphone 45 and the transmitter in portable unit 10. Conductor37 provides an electrical connection between switch 47 and thetransmitter in portable unit 10. Switch 47 when actuated by actuationlever 49, secured to a side wall of housing 15, energizes thetransmitter in portable unit 10. Conductor 38 provides an electricalconnection between .the receiver in portable unit 10 and speaker 46.

Cable 27 enters housing 15 through aperture 50 in bottom wall 17, and isretained by cable clamp 51, secured to the inner surface of housing 15.Center conductor 29 of the coaxial transmission line is connected toconnector 25 for providing an electrical interconnection between thetransmitter and receiver in portable unit 10 and helical antenna element22. Outer conductor 31 of the coaxial transmission line in cable 27 isconnected, through connector 12, to ground potential in portable unit10, and has its other end terminated at a first point 53 in housing 15.This first point is connected via conductor 54 to frame 48 of speaker46, and to microphone 45 and switch 47 for providing ground potentialsthereat. First point 53 is also connected to one terminal of speaker 46through RF choke 52 to provide a ground potential for the speaker. Choke52 acts to prevent RF energy from being coupled to speaker 46.

The second outer conductor 34 extends from first point 53 to a point 54along cable 27 which is external to housing 15. Outer conductor 31 ofthe coaxial transmission line, insulator 32, and second outer conductor34 form a transmission line which acts as the second portion of thedipole antenna in assembly 11. This second portion acts to form asubstantial ground or reflecting plane, which is necessary in order toprovide the desired radiation pattern for the dipole antenna. Thephysical length L, of second outer conductor 34 is selected such thatthe electrical length of the transmission line formed by conductor 31,insulator 32 and conductor 34 is substantially equal to a quarterwavelength at the resonant frequency of the dipole antenna.

Second outer conductor 34 is electrically connected to outer conductor31 at first point 53 to form a low impedance feed point for the dipoleantenna, The second end of second outer conductor 34, shown at 54 isterminated without connection to other elements in cable 27 in order toprovide an open circuit at that end of the 1 dipole. Second end 54 ofsecond outer conductor 34, as it is open circuited or unterminated, actsas a termination for the second portion of the dipole antenna,preventing radiated energy from being conducted along cable 27 towardsportable unit 10. The open circuit also prevents undesired radiatedenergy from being conducted along cable 27 to the antenna from portableunit 10.

As feed point 53 is a low impedance feed point for the dipole antenna,external capacitance, added at or near this point will not substantiallydegrade the antenna performance. Housing 15 is connected to the feedpoint of the antenna. A user, holding housing 15 of speaker microphoneassembly 11 in his hand will then be holding the dipole antenna, at ornear the feed point 53. The user's body capacitance will therefore havea minimal effect upon the radiation characteristics of the dipoleantenna. Components such as microphone 45, switch 47, and speaker 46,particularly speaker frame 48, because of its mass, can seriously impairantenna radiation characteristics due to their presence in the nearfield produced by the radiation. By connecting the speaker frame 48 toground potential at feed point 53, and the other components to thispoint, their affects upon the radiation characteristics are minimized.

Helical antenna element 22, due to its design, has a low inputimpedance. Transmitters and receivers commonly used in portable radiounits, such as unit 10, have an output impedance of approximately 50ohms. The transmission line consisting of second outer conductor 34,insulator 32 and outer conductor 31, in addition to acting as the groundportion of the dipole antenna, also provides an impedance match formatching the input impedance of antenna element 22 to the outputimpedance of the radio equipment. This is due to the electrical lengthof second conductor 34. By using the grounded portion of the dipoleantenna as an impedance matching device, discrete matching components,normally required for matching a transmitter output of one impedance toa radiating element of a different impedance, are no longer required.

Referring to FIG. 4, antenna element 60 is the same as antenna element22 in FIG. 3, except that threaded rod 23 is replaced by an RF connector61. Housing 65 has a top wall 66, a bottom wall 67 and side walls, justas housing in FIG. 3. Helical antenna 60 is detachably secured tohousing 65 by RF connector 69 mounted to top wall 66 of housing 65.Cable 70 extends through an aperture 71 in bottom wall 67 of housing 65and provides an electrical connection between the speaker microphoneassembly and a portable radio unit such as portable radio unit 10.

Cable 70 includes a coaxial transmission line consisting of a centerconductor 73, a dielectric insulating material 74, a braided shield orouter conductor 75 and an outer insulation 76. Conductors 77, 78 and 79are also provided in cable 70 for interconnecting the components locatedin housing 65 with theequipment in the portable radio unit. A flexiblemetal tubing 80 commonly known as an armored cable, encases conductors77, 78 and 79 and the transmission line. Armored cable 80 is used inorder to prevent the speaker microphone assembly from being broken awayfrom the portable radio unit. A plastic tubing 81 encases armored cable80.

Housing 65 has mounted therein a microphone 82, a

speaker 83, a transmitter actuation switch 84, and a speaker high-lowswitch 85. An actuation lever 86 is mounted to the side wall of housing65, adjacent transmitter actuation switch 84 for operating this switch.Conductors 77, 78 and 79 provide electrical interconnections betweenthese components and the portable unit.

Cable 70 enters housing 65 through aperture 71 and is retained by cableclamp 88 secured to the inner and outer surfaces of bottom wall 67.Center conductor 73 of the coaxial transmission line is connected to RFconnector 69 for providing an electrical interconnection between thetransmitter and receiver in the portable unit and helical antennaelement 60. Outer conductor 75 of the coaxial transmission line in cable70 is connected through a connector, such as connector 12 in FIG. 1, toground potential in the portable unit, and has its other end terminatedat a first point 89 in housing 65. The first point in this embodiment isthe outer shell of RF connector 69. This first point is connected viaconductor 90 to speaker magnet housing 93, via conductor 91 to point 94on the inner surface of housing 65, and via RF choke 92 to one terminalof microphone 82 and speaker 83.

The inner surfaces of the walls of housing are coated with a conductivemetal paint. In the embodim'ent shown, a conductive silver paint isapplied having a thickness of at least 1 mil. A thickness of l to 3 milsinsures that the painted conductive surface acts as a good conductorover the frequency ranges in which the portable unit operates. The 1 to3 mil thickness coating will allow efficient operation at frequencies upto and in excess of 470 MHz. At aperture 71 surrounding cable clamp 88on the inner surface of bottom wall 67, the conductive paint is notapplied for reasons to be explained in a subsequent portion of thisapplication.

Housing 65, because of the conductive paint applied to the inner surfaceof the walls, acts in the same manner as second outer conductor 34 shownin FIG. 3. That is, as it surrounds the coaxial transmission line and isitself conductive at the frequencies of interest, the painted surfacealong with transmission line outer conductor 75, forms a secondtransmission line which acts as the second portion or grounded portionof the dipole antenna. Although not physically as long as second outerconductor 34, the physical dimensions of the housing in width, heightand thickness, cause it to be resonant at the approximate operatingfrequencies of interest. The conductive inner surface of housing 65 iselectrically connected at first point 89 via RF connector 69, viaconductor 91, and the grounded metal portions of the components inhousing 65, to form a low impedance feed point for the dipole antenna.Aperture 71 through which cable passes, as previously stated, is notprovided with a conductive coating. This allows aperture 71 to act in amanner equivalent to the second end of second outer conductor 34 in FIG.3. With no silver conductive paint applied, aperture 71 acts to providean open circuit at the bottom surface of housing 65. This open circuitis a high impedance and acts as a termination for the second portion ofthe dipole antenna, preventing radiated energy from being conductedalong cable 70 to the antenna from portable unit 10.

As feed point 89 is a low impedance feed point for the dipole antenna,and the housing is a part of this low impedance feed point, externalcapacitance added at or near this point will not substantially degradethe antenna performance. A user holding housing 65 in his hand will thenbe holding the dipole antenna at or-near the feed point, and the user'sbody capacitance will therefore have a minimal affect upon the radiationcharacteristics of the dipole antenna. As all the components mountedwithin housing 65 are also connected to the feed point, their affectsupon the radiation characteristics are minimized.

Housing 65 also acts to match the impedance of antenna element 60 to theimpedance of the transmitter and receiver in the portable radio unit inthe same manner as that previously described with respect to secondouter conductor 34 in FIG. 3.

As can be seen, a speaker microphone assembly has been provided whichincludes a dipole antenna for radiating and transmitting signals. Thedipole antenna provides an improved radiation pattern over antennaspreviously used. By providing a dipole antenna, the reflecting planeconstitutes a substantial ground plane for improving the radiationpattern from the radiating element. With the dipole antenna feed pointmounted in the speaker microphone assembly housing, body capacitance ofthe user will not substantially affect or degrade the antennaefficiency. Furthermore, a quarter wavelength coaxial transmission lineor its equivalent such as is provided by the housing in FIG. 4, whenused as the reflecting portion of the dipole also provides an impedancematch so that critical matching components are not required to match thetransmitting and receiving equipment to the antenna radiating element.The quarter wavelength coaxial transmission line or its equivalent suchas is provided by the housing in FIG. 4, also prevents unwanted RFenergy from being conducted to or from the portable unit.

We claim:

1. In a speaker microphone assembly adapted to be held in the hand andfor use with a portable radio unit including receiving and transmittingequipment for receiving and transmitting signals, the combination of, ahousing having top, bottom and side walls, an elongated antenna elementsecured to said top wall, said antenna element forming one half of adipole antenna for radiating signals coupled from said transmittingequipment and coupling radiated signals to said receiving equipment, acable for coupling the portable unit to said housing, said cablecontaining a coaxial transmission line having an inner conductor and anouter conductor, said outer conductor terminating at a first point insaid housing, circuit means coupling said inner conductor to saidantenna element, a second outer conductor surrounding said coaxialtransmission line and extending for a predetermined length from saidfirst point 7 along saidtransmission line, said second outer conductorbeing connected to said outer conductor of said transmission line atsaid first point and both conductors forming the other half of saiddipole antenna.

2. The assembly of claim 1 wherein said dipole antenna is resonant at aparticular frequency, said antenna element and said other half of saiddipole antenna each having an electrical length substantially equal to aquarter wavelength at the resonant frequency of said dipole antenna.

3. The assembly of claim 2 wherein said antenna element includes aconductive element having first and second ends, and mounting meanssecured to one of said ends and adapted to engage said circuit means insaid assembly.

4. The assembly of claim 3 wherein said conductive element is formedinto a helix for providing a quarter wavelength helical antenna element.

5. The assembly of claim 4 wherein said antenna element includes a tubeof nonconductive material surrounding said helical element, saidnonconductive material acting to insulate said helical element andmaintain the same in a semi-rigid condition.

6. The assembly of claim 4 wherein said circuit means is a connector,said connector being secured to a wall of said housing and having saidcenter conductor coupled thereto, said connector adapted to mate withsaid antenna element mounting means for mounting said antenna element tosaid assembly housing.

7. The assembly of claim 6 wherein said cable further includesinsulating means surrounding said outer conductor of said coaxialtransmission line and between said outer conductor and said secondconductor, said insulating means providing an insulated separationtherebetween.

8. The assembly of claim 7 wherein said portable unit includes a powersource having first and second terminals, one of said terminals forminga reference potential, said reference potential being coupled to saidtransmission line outer conductor.

9. The assembly of claim 8 further including speaker means, microphonemeans and unit actuating means mounted in said assembly housing,conductor means coupling said speaker means, microphone means andactuating means to the portable unit for providing electricalconnections therebetween, said speaker means including a speaker frame,said speaker means, said speaker frame and said microphone means beingcoupled to said first point to provide a reference potential thereat.

10. The assembly of claim 9 wherein said outer conductor of saidtransmission line and said second outer conductor are formed frombraided wire conductors.

11. The assembly of claim 9 wherein said second outer conductor is aconductive material secured to said housing walls and coupled to saidfirst point, said housing further including an aperture in said bottomwall, said cable extending through said aperture and said conductivematerial being isolated from said aperture.

12. The assembly of claim 11 wherein said conductive material isaconductive paint applied to said housing innerwalls and having apredetermined thickness.

13. The assembly of claim 12 wherein said conductive paint is a silverconductive paint.

14. The assembly of claim 13 wherein said predetermined thickness is atleast 1 mil.

15. A speaker microphone assembly for use with a portable radio unitincluding receiving and transmitting equipment for receiving andtransmitting signals, said portable radio unit having a power sourcetherein with one terminal thereof being a reference potential, saidspeaker microphone assembly including in combination, a housing havingtop, bottom and side walls adapted to be held in a users hand, saidbottom wall having an aperture therethrough, transducer means mounted insaid assembly housing, a helical antenna element detachably secured tosaid housing, said antenna element forming one portion of a dipole.antenna for radiating signals coupled from said transmitting equipmentand coupling radiated signals to said receiving equipment, a connectorsecured to said housing for detachably securing said helical antennaelement, a cable for coupling the portable unit to said housing, saidcable containing a plurality of conductors coupling said equipment tosaid transducer means for providing electrical connections therebetween,and a coaxial transmission line having an inner conductor and an outerconductor, said inner and outer conductor of said coaxial transmissionline being connected to said connector, said inner conductor couplingsaid helical anconductor and said second outer conductor togetherforming the other portion of said dipole antenna.

16. A speaker microphone assembly of claim 15 wherein said dipoleantenna is resonant at a particular frequency, said antenna elementhaving an electrical length substantially equal to a quarter wavelengthat the resonant frequency of said antenna.

1. In a speaker microphone assembly adapted to be held in the hand andfor use with a portable radio unit including receiving and transmittingequipment for receiving and transmitting signals, the combination of, ahousing having top, bottom and side walls, an elongated antenna elementsecured to said top wall, said antenna element forming one half of adipole antenna for radiating signals coupled from said transmittingequipment and coupling radiated signals to said receiving equipment, acable for coupling the portable unit to said housing, said cablecontaining a coaxial transmission line having an inner conductor and anouter conductor, said outer conductor terminating at a first point insaid housing, circuit means coupling said inner conductor to saidantenna element, a second outer conductor surrounding said coaxialtransmission line and extending for a predetermined length from saidfirst point along said transmission line, said second outer conductorbeing connected to said outer conductor of said transmission line atsaid first point and both conductors forming the other half of saiddipole antenna.
 1. In a speaker microphone assembly adapted to be heldin the hand and for use with a portable radio unit including receivingand transmitting equipment for receiving and transmitting signals, thecombination of, a housing having top, bottom and side walls, anelongated antenna element secured to said top wall, said antenna elementforming one half of a dipole antenna for radiating signals coupled fromsaid transmitting equipment and coupling radiated signals to saidreceiving equipment, a cable for coupling the portable unit to saidhousing, said cable containing a coaxial transmission line having aninner conductor and an outer conductor, said outer conductor terminatingat a first point in said housing, circuit means coupling said innerconductor to said antenna element, a second outer conductor surroundingsaid coaxial transmission line and extending for a predetermined lengthfrom said first point along said transmission line, said second outerconductor being connected to said outer conductor of said transmissionline at said first point and both conductors forming the other half ofsaid dipole antenna.
 2. The assembly of claim 1 wherein said dipoleantenna is resonant at a particular frequency, said antenna element andsaid other half of said dipole antenna each having an electrical lengthsubstantially equal to a quarter wavelength at the resonant frequency ofsaid dipole antenna.
 3. The assembly of claim 2 wherein said antennaelement includes a conductive element having first and second ends, andmounting means secured to one of said ends and adapted to engage saidcircuit means in said assembly.
 4. The assembly of claim 3 wherein saidconductive element is formed into a helix for providing a quarterwavelength helical antenna element.
 5. The assembly of claim 4 whereinsaid antenna element includes a tube of nonconductive materialsurrounding said helical element, said nonconductive material acting toinsulate said helical element and maintain the same in a semi-rigidcondition.
 6. The assembly of claim 4 wherein said circuit means is aconnector, said connector being secured to a wall of said housing andhaving said center conductor coupled thereto, said connector adapted tomate with said antenna element mounting means for mounting said antennaelement to said assembly housing.
 7. The assembly of claim 6 whereinsaid cable further includes insulating means surrounding said outerconductor of said coaxial transmission line and between said outerconductor and said second conductor, said insulating means providing aninsulated separation therebetween.
 8. The assembly of claim 7 whereinsaid portable unit includes a power source having first and secondterminals, one of said terminals forming a reference potential, saidreference potential being coupled to said transmission line outerconductor.
 9. The assembly of claim 8 further including speaker means,microphone means and unit actuating means mounted in said assemblyhousing, conductor means coupling said speaker means, microphone meansand actuating means to the portable unit for providing electricalconnections therebetween, said speaker means including a speaker frame,said speaker means, said speaker frame and said microphone means beingcoupled to said first point to provide a reference potential thereat.10. The assembly of claim 9 wherein said outer conductor of saidtransmission line and said second outer conductor are formed frombraided wire conductors.
 11. The assembly of claim 9 wherein said secondouter conductor is a conductive material secured to said housing wallsand coupled to said first point, said housing further including anaperture in said bottom wall, said cable extending through said apertureand said conductive material being isolated from said aperture.
 12. Theassembly of claim 11 wherein said conductive material is a conductivepaint applied to said housing inner walls and having a predeterminedthickness.
 13. The assembly of claim 12 wherein said conductive paint isa silver conductive paint.
 14. The assembly of claim 13 wherein saidpredetermined thickness is at least 1 mil.
 15. A speaker microphoneassembly for use with a portable radio unit including receiving andtransmitting equipment for receiving and transmitting signals, saidportable radio unit having a power source therein with one terminalthereof being a reference potential, said speaker microphone assemblyincluding in combination, a housing having top, bottom and side wallsadapted to be held in a user''s hand, said bottom wall having anaperture therethrough, transducer means mounted in said assemblyhousing, a helical antenna element detachably secured to said housing,said antenna element forming one portion of a dipole antenna forradiating signals coupled from said transmitting equipment and couplingradiated signals to said receiving equipment, a connector secured tosaid housing for detachably securing said helical antenna element, acable for coupling the portable unit to said housing, said cablecontaining a plurality of conductors coupling said equipment to saidtransducer means for providing electrical connections therebetween, anda coaxial transmission line having an inner conductor and an outerconductor, said inner and outer conductor of said coaxial transmissionline being connected to said connector, said inner conductor couplingsaid helical antenna element to said equipment, said outer conductorforming a low impedance point at said connector, said housing furtherincluding a conductive material applied to said housing inner walls andconnected to said low impedance point, said conductive material beingisolated from said aperture in said bottom wall and forming a secondouter conductor for surrounding said coaxial Transmission line, saidtransmission line outer conductor and said second outer conductortogether forming the other portion of said dipole antenna.